It is generally recognized that when a filtration process or apparatus is evaluated in terms of treatment performance, the filter bed assumes primary importance. This is especially true for filtration processes or units with deep beds composed of several layers of differing filter media.
The continued efficient performance of filtration units requires the periodic cleaning of the filter media. Thorough cleaning of the filter deck and media requires a very rapid and uniformly distributed passage of fluids, such as water or air, through the bed for several reasons. First, the filter media must be completely fluidized so that the agitation of the particles will effect complete detachment of the dirt particles from the media particles. Second, the fluid flow through the bed must have identical vertical velocities over the entire cross-sectional area of the filter and thus can only be obtained by obtaining a uniform drop in fluid pressure over the filter bed. Different fluid velocities at different points in the filter bed can cause the media particles to rise at those points in the filter bed with greater vertical velocities than at other points and this can (a) result in the undesirable removal of the media particles by entrainment with the wash fluid, and (b) if the particles of the various filter media used do not rise uniformly, upon completion of the backwashing they may not settle in proper sequence and to the proper depth and this will prevent uniform horizontal distribution of the various media over the entire cross-sectional area of the filter when backwashing is resumed. This problem is particularly acute in the case of filters having large cross-sectional areas.
Many attempts have been made to achieve a uniform flow, i.e., equal vertical velocities and fluid distribution, throughout the area of a filter bed but none has been completely successful mainly because of the difficulty of establishing and maintaining a uniform fluid pressure over the entire area of the bed. Initially even a small increase in the vertical fluid flow through a portion of the filter deck at some point will result in imbalanced pressure drops which will result in progressively increased fluid flow through that particular section of the bed. This would tend to leave the other portions of the filter media beds not completely fluidized and consequently unwashed. Porous plates which support the filter media become clogged with fine particles and non-uniform flow and the conditions described above result. Nozzles with various shapes extending into the media and with either gravel or balls surrounding their open discharge ends have not achieved the desired uniform flow as they cause concentrated fluid streams which displace the media unevenly. Hooded nozzles discharging narrow fluid streams through fine slots are not effective when the narrow slots become obstructed with particles and non-uniform flow through the filter deck results, and thus usually leads to a progressive increase in resistance to flow through and a sharp drop in the capability of this filter.